The present invention concerns a process for the production and improved separation of aromatic C8 hydrocarbons from a feed which is rich in para-xylene, such as a C8 cut from the Mobil toluene disproportionation process.
It has long been recognized that certain zeolitic materials catalyst certain hydrocarbon conversions as described, for example, in United States patent U.S. Pat. No. Particular hydrocarbon conversions which have been claimed are alkylation, transalkylation and disproportionation. Toluene disproportionation is claimed, for example, in United States patents U.S. Pat. No. 4,052,476; U.S. Pat. Nos. 4,007,231; 4,011,276; 4,016,219 and 4,029,761.
In their article in Oil and Gas Journal vol. 69, No 48 (1971), Grandio et aL describe a process for the disproportionation of toluene in the liquid phase using zeolitic catalysts in the absence of hydrogen.
It should be noted that in the first known processes, the composition of the xylenes obtained corresponds to the thermodynamic equilibrium at the operating temperature, namely 20% to 25% of p-xylene of the total of xylenes and ethylbenzene.
More recent patents, for example U.S. Pat. No. 4,380,685, describe alkylation, transalkylation and paraselective disproportionation of substituted aromatic compounds to form dialkylbenzenes. In the case of toluene disproportionation, the p-xylene concentration in the effluent produced exceeds the thermodynamic equilibrium. The catalyst used is based on zeolites characterized by a constraint index of 1 to 12, a silica/alumina ratio of at least 12/1 and comprising various metals and phosphorous; examples of the zeolites used are ZSM5, ZSM11, ZSM12 and ZSM35. Further, those catalysts must be treated to restrict diffusion of ortho- and meta-xylene through the pores of the crystal and to reduce re-isomerization of the p-xylene formed. This treatment essentially consists of precoking under well defined conditions which may be accompanied by addition of a small quantity of an oxide which is difficult to reduce (antimony, phosphorous, boron or magnesium), or by a surface treatment. Examples of methods for the preparation of such an effective catalyst are described in U.S. Pat. No. 5,173,461.
The process using those catalysts, termed the MSTDP process, has been described in two publications:
Selective toluene disproportionation process proven at Italian refinery, Gorra F., Breckenridge L. L., Guy W. M., Sailor R. A., Oil and Gas Journal Vol. 90, No. 41, 60-67 (1992) and in PA1 "Mobil's toluene to PX process proves itself", Mobil Research and Development Corp., European Chemical News Vol. 54, No 1418 (1990). PA1 a) a paraselective toluene disproportionation step in the presence of hydrogen and a catalyst in a disproportionation zone to produce an effluent containing benzene, toluene, para-xylene enriched xylenes, ethylbenzene and aromatic hydrocarbons containing at least 9 carbon atoms; PA1 b) an effluent distillation step using at least two distillation columns to separately produce benzene, toluene and xylenes; PA1 c) a high temperature xylene crystallization step, preferably in the range +10.degree. C. to -30.degree. C., using at least one crystallization zone to produce very high purity para-xylene and a mother liquor which is depleted in para-xylene, the process being characterized in that: PA1 d) the mother liquor is brought into contact with a zeolitic adsorption bed in a simulated moving bed adsorption zone in the presence of a toluene desorbent under conditions such that a first fraction is obtained which is depleted in para-xylene and contains toluene and a second fraction is obtained which is enriched in para-xylene and contains toluene; PA1 e) at least a portion of the second fraction is recycled to the crystallization zone after distilling substantially all of the toluene. PA1 In a first variation, the xylenes are crystallized in accordance with step (c) in a crystallization zone to produce a suspension of para-xylene crystals in the mother liquor, the crystals are separated from the mother liquor in a separation zone, the crystals are washed with a suitable wash solvent, the mother liquor is recovered and sent to the adsorption zone in accordance with step (d) and very high purity para-xylene crystals are recovered, and the wash liquor is recovered and recycled to the distillation zone if the wash solvent contains toluene or to the crystallization zone or the adsorption zone if the wash solvent is molten para-xylene. PA1 In a second variation which may be preferred with certain types of crystallizer, for example scraped surface crystallizers, the xylenes are crystallized in accordance with step (c) in at least two crystallization zones, one of which is at a colder temperature than the other, the mother liquor from the coldest crystallization zone being recovered and sent to the adsorption zone in accordance with step (d).
These publications describe the production of xylenes containing 80-95% of p-xylene for a conversion of 30% of toluene per pass.
A number of patents claim this process and describe catalyst preparations. Examples are European patent EP 26,962, U.S. Pat. Nos. 4,260,843; 4,274,982; 4,908,342; 5,173,461; and WO 93/17987.
In all cases, the disproportionation effluent has a simplified composition of close to: 70% toluene--15% benzene--15% xylenes. The composition of the xylene fraction is 85% p-xylene, 15% o-+m-xylene and ethylbenzene.
The present invention relates to an improved process for the separation of a C8 aromatic mixture to obtain p-xylene of sufficient purity for the production, for example, of terephthalic acid. One known method for the separation of p-xylene from a C8 aromatic cut consists of carrying out fractional crystallization; examples of existing processes are those or Chevron, Krupp, Amoco, Maruzen and Arco (U.S. Pat. No. 3,177,255 and U.S. Pat. No. 3,467,724). Those processes use the following procedure: the feed containing at least 20% of p-xylene is cooled to between 50.degree. C. and 70.degree. C. to cause crystallization, the crystals containing 85% to 90% of p-xylene and the mother liquors, 7% to 8%; the crystals are melted again and recrystallised at 10.degree. C. After filtering the crystals and washing with toluene, for example, 99.5% pure p-xylene is obtained.
A further method for the separation of p-xylene from an aromatic C8 mixture is the liquid chromatographic method termed a simulated counter-current method claimed in U.S. Pat. No. 2,985,589 which uses zeolites to selectively adsorb the p-xylene. The Parex and Aromax processes use this method or a simulated co-current method (U.S. Pat. No. 4,402,832.
The advantages and disadvantages of those processes have been widely studied and described for different cases.
The prior art is also illustrated in U.S. Pat. No. 5,329,060 and EP-A-0 531 191 which describe a combination of adsorption, crystallization and isomerization steps for aromatic C8 cuts to produce very high purity para-xylene.
The peculiarity of the mixture obtained at the outlet from the paraselective disproportionation reactor in the MSTDP, after separation of the lighter hydrocarbons by distillation, is that it is particularly rich in p-xylene as it contains 75% to 85% of p-xylene.
This mixture can be treated in different ways:
1. High temperature crystallization PA0 2. Two-temperature crystallization
This means a temperature which is, for example, in the range +10.degree. C. to -25.degree. C. The yield of pure p-xylene is close to 83% when the concentration of p-xylene in the C8 cut in the effluent is 80 weight %. This yield is limited by the fact that the mother liquor still contains more than 40% of para-xylene due to the liquid-solid thermodynamic equilibrium.
In this case, a first crystallization at a temperature in the range +10.degree. C. to -25.degree. C. leads to the production of pure para-xylene, the mother liquor is recrystallized at a lower temperature (-50.degree. C. to -70.degree. C.), the p-xylene-rich crystals are recycled to the feed to the first crystallization, the mother liquor from the second crystallization contains no more than about 10 weight % of p-xylene.
The global yield of para-xylene from the two-stage crystallization method for an initial feed of 80 weight % of para-xylene is about 97 weight %.
The disadvantage of this process is high energy consumption.